Biological invasions are natural experiments in community assembly. The introduction of spotted knapweed (Centaurea stoebe) in the intermountain grasslands of western Montana has resulted in a dramatic reassembly of the native web-building spider community. Web-spiders are typically limited by the number of available web substrates in native grasslands. Knapweed invasion appears to release spiders from this ecological constraint, causing dramatic increases in population abundances in invaded areas (up to 74x). Here we explore the specific mechanisms driving web-spider community reassembly following knapweed invasion, focusing on the effects of plant architecture. We evaluated this question by creating a large-scale, replicated experiment that simulates the invasion of only knapweed’s plant architecture in native (uninvaded) grasslands. At each site two 50 x 50 m plots were established – one plot received a simulated knapweed treatment while the other plot served as a control with no stems introduced. Plots were cleared of all web-spiders and seeded with known abundances of the three dominant web-spiders. Community assembly was evaluated by observing spider abundance over 2 summers following “invasion”. Specific mechanisms facilitating this assembly were also evaluated, namely web size, number and size of prey captured, and reproductive output.
Results/Conclusions
In simulating the architectural attributes of a spotted knapweed invasion, we have observed the native web-building spider community reassembling in a pattern similar to those following natural spotted knapweed invasions. This result establishes that spotted knapweed architecture is the invader attribute that causes the spider community to reassemble. We have also found that species-specific differences in web construction in response to the simulated invasion was the primary mechanism that determined how strongly different spider groups responded to the invasion by influencing prey capture rates, fecundity, and particularly substrate limitation.
The architectural changes associated with spotted knapweed invasion appear to release one group of spiders (Dictyna) from web-substrate constraints far more than others (orb weavers). Dictyna build their webs within a single substrate, while orb weavers use multiple substrates upon which to string their webs. Because of this, Dictyna are much more ecologically constrained by the native system than orb-weavers, because the native plants suitable for Dictyna are much smaller, more ephemeral, and occur at much lower densities than spotted knapweed. These results demonstrate both why and how native web-spiders reassemble following spotted knapweed invasion and also how biological invasions can be used as natural experiments in community assembly.